J. Evans writes:
The construction of a valve is not a completely rigid structure. Any relative movement of the various electrodes not only affect sinter-electrode capacitances but also variations in current and mutualconductance. Such relative movements can be caused by vibration, such as sudden knocks or even ambient sounds. Mullard presented the results of an investigation into micrphonics in valves which it published in two articles that appeared in the November 1962 issue of their publication "Mullard Technical Communications"
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